The present invention relates to energetic plasticizers and, in particular, to compositions of formal-based energetic plasticizers for using in high performance, insensitive, plastic-bonded explosive (PBX) charges.
Explosives having a very high rate of reaction and high-pressure development are often useful in payloads of a wide variety of military armaments. The technical specifications for such a charge are varied, and relate to parameters including effective combat performance, maximum blast wave overpressure, blast speed and total blast energy, and safety qualities and durability under widely-varying environmental conditions.
One of the main requirements in the development of new explosive charges is to improve their safety characteristics while preserving their performance. In order to do this the warhead contains a powerful secondary explosive together with a binder and plasticizer. The identity and weight fraction of the binder and plasticizer can be changed to control the characteristics of the resulting explosive formulation.
The performance of the explosive charge is of particular importance when the charge is used in self-forging fragments (SFF), or shaped-charge (also known as hollow-charge) warheads. These warheads are used to penetrate hard targets, mainly military vehicle armor and fortifications. In such formulations the use of energetic plasticizers is of great advantage in view of the high weight fraction of plasticizers—typically 2-9% in the shaped charge.
One of the most severe limitations on storage and operation of shaped-charge warheads relates to the glass transition temperature (Tg) of the material. Below the Tg, the charge is brittle. Moving the warhead when the temperature is below the Tg can cause cracks in the explosive charge, leading to a substantial reduction in charge performance.
The glass transition is a phase transition that occurs in amorphous polymers, and is intuitively analogous to the phenomenon of melting that occurs in crystalline polymers. Below the Tg, the long-range motion of the polymer chains is stopped, and the polymer becomes hard, fragile, and breakable. Therefore, it is desirable to use a binder/plasticizer composition that has a low Tg value. However, the most common approach to the development of new energetic plasticizers is to consider the melting point of the plasticizer itself. It is well known in the art that using specific weight ratios of the energetic plasticizer components can reduce the melting temperature.
Various energetic plasticizers are known in the art. U.S. Pat. No. 4,997,499 to Adolph reports a known energetic plasticizer, a 1:1 eutectic mixture of bis(2,2-dinitropropyl) formal (BDNPF) and bis(2,2-dinitropropyl) acetal (BDNPA). However, the use of BDNPA was found to be disadvantageous, because of the limited chemical and thermal stability of BDNPA.
U.S. Pat. No. 4,997,499 to Adolph teaches an energetic plasticizer containing a 1:1 molar binary eutectic mixture of bis(2,2-dinitropropyl) formal (BDNPF) and 2,2-dinitrobutyl 2,2- dinitropropyl formal (DNBPF). It is noted that the 1:1 molar ratio of bis(2,2-dinitropropyl) formal (BDNPF) to 2,2-dinitrobutyl 2,2-dinitropropyl formal (DNBPF) produces the mixture with the lowest melting point. Although the ratio of BDNPF to DNBPF may be varied from 1:1, U.S. Pat. No. 4,997,499 to Adolph emphasizes that this results in a corresponding rise in the melting point, such that there is little, if any, advantage in doing so.
In practice, the product mixture produced in the synthesis of the eutectic mixture is composed of BDNPF, DNBPF and bis(2,2-dinitrobutyl) formal (BDNBF) in a molar ratio of 1:1:0.05, respectively. The small amount of bis(2,2-dinitrobutyl) formal present in the product does not interfere with the performance of the mixture as a thermally and chemically stable energetic plasticizer.
Cho, et al., in “An Improved Mixed Formal Energetic Plasticizer” (NDIA Conference, IM-EM, November 1999, Tampa, Fla., pp 404-413) verify that the BDNPF/DNPBF/BDNBF mixed formal disclosed by U.S. Pat. No. 4,997,499 to Adolph is characterized by good performance and superior physical properties with respect to the widely used BDNPF/BDNPA energetic plasticizer compositions.
It is emphasized by U.S. Pat. No. 6,592,692 to Cho, et al., that the synthesis taught by U.S. Pat. No. 4,997,499 to Adolph is plagued by the formation of an undesirable side product, bis(2,2-dinitropropyl) diformal. Although Cho, et al., in “An Improved Mixed Formal Energetic Plasticizer”, disclose methods of reducing the amount of diformal side product produced, these methods are cumbersome and expensive, can involve additional process steps, and do not completely eliminate the formation of the diformal.
Moreover, U.S. Pat. No. 6,592,692 to Cho, et al., further notes that despite the superior thermal and chemical properties and the apparent low cost of the above-described mixed formal, no process for producing the mixed formal has been implemented in industry. U.S. Pat. No. 6,592,692 attributes this to the complexity of the process, and to the need for another precursor (2,2-dinitrobutanol) in addition to the 2,2-dinitropropanol precursor used in other processes (and hence requires an additional synthesis process).
U.S. Pat. No. 6,592,692 to Cho, et al., goes on to teach a plasticizer mixture of formals having a low melting point and containing bis(2,2-dinitropropyl) formal (BDNPF) and bis(2,2-dinitropropyl) diformal (BDNPDF).
U.S. Pat. No. 6,620,268 to Cho, et al., discloses a plasticizer mixture of formals having a eutectic mixture of BDNPF, DNBPF and BDNBF. The mixtures appear to be of a very similar composition to those obtained by U.S. Pat. No. 4,997,499 to Adolph in the laboratory and described hereinabove, i.e., BDNPF, DNBPF and BDNBF in a molar ratio of 1:1:0.05, respectively. In any event, it is explicitly taught by U.S. Pat. No. 6,620,268 to Cho, et al., that the preferable molar ratio of the BDNPF/DNPBF/BDNBF contained in the plasticizer is in the range of 20-68%/28-50%/4-30%. The plasticizer of the present invention may further contain bis(2,2-dinitropropyl) diformal by less than 5%, preferably by less than 3%, and most preferably by less than 1%.
The advantages of including BDNBF in the plasticizer are not elaborated. Indeed, it might be understood from U.S. Pat. No. 6,620,268 that BDNBF is simply a synthesis side product that, as articulated by U.S. Pat. No. 4,997,499 to Adolph, “does not interfere with the performance of the mixture as a thermally and chemically stable energetic plasticizer”. The BDNBF content with respect to the total formal content is 4.3-8.9 mole % in the four examples provided in U.S. Pat. No. 6,620,268, all of which are strikingly similar to the 5 mole % obtained by U.S. Pat. No. 4,997,499 as a side product in the synthesis of BDNPF and DNBPF.
There is therefore a need for an energetic plasticizer that exhibits superior chemical and thermal stability, and is simple and inexpensive to produce.